Excellent thread and great discussion. I can't add anymore to the excellent links everybody provided, but I'd like to add some of my personal observations regarding outlfow boundaries and supercells.
I like good, strong outflow boundaries that a stationary or very slow moving and a result of overnight or early morning extgensive convection (small to moderate MCS are excellent) that dissipates or moves away to allow good insolation on either side of the boundary. These types of boundaries stand up well against strong surface flow throughout the day.
These preferred boundaries are almost always able to be analyzed on surface charts with distinct temperature and/or dewpoint differences and especially wind vectors. They are almost synoptic in nature like a cool/warm front. Sometimes, the difference on a surface chart is subtle like a 5F temp or dewpoint difference and subtle windshift. Overall, my general rule of thumb is that if the boundary shows up distinctly on radar and/or satellite and persists into th early afternoon, it's a boundary to be taken seriously. If I can't find it on radar/satellite as well as careful surface analysis, then I don't give it a second look.
Sometimes though, outflow boundaries, even strong ones, get diffused by strong surface flow and end up dissipating entirely by initiation. I find this true alot during the early part of the season where you get these crazy 50-60 knot 850mb jets with related surface winds.
Sometimes, there are things that look like outflow boundaries like a gravity wave..especially one trapped under a strong cap. These can easily fool you in a pre-afternoon analysis, so check the surface plots carefully. These things can lead you astray and look impressive only to completely disappear by early afternoon.
Of course, anytime you can get a dryline, cool front, or another outflow boundary intersecting it, then perform the "Forbidden Sacred Dance of Chaser Merriment" 8)
That's my $0.02 worth anyway.